1. Field of the Invention
The present invention relates to a cellulose degradable yeast and a production method therefor.
2. Description of the Related Art
Fermentation microorganisms that originally cannot metabolize principal components of soft-biomass, such as cellulose, hemicellulose, and the like have been modified using bioengineering methods as an attempt to attain ethanol fermentation directly from non-edible carbon sources. Cell surface-displaying techniques are suitably used as such bioengineering methods. For example, yeasts that display on the surface a group of enzymes (i.e., a plurality of enzymes) that hydrolyze cellulose have been produced by cell surface-displaying techniques (for example, WO 01/79483 and Japanese Laid-Open Patent Publication No. 2008-86310).
Originally, organisms regulate the level of protein expression in various processes such as transcription, translation, and the like. However, it is very difficult to control the levels and balance of expression of a plurality of foreign proteins in the foreign gene expression system of a yeast with genetically modified.
In conventional researches, it has been common that the expression level of foreign-gene is regulated by selecting the type of promoter. However, such a regulation depends on various factors such as the gene to be expressed, the kind of host yeast strain, the culture conditions, and like, making precise control or extensive application difficult, and it has not been possible to determine the optimal expression level.
It is known that, for example, a filamentous fungus Trichoderma reesei or the like, which is known to efficiently degrade cellulose, retains a very large variety of cellulase genes and precisely controls the balance of their expression depending on the environment. However, when those genes are heterologously expressed in different microorganisms such as yeast, it is very difficult to control the balance of their expression.
The δ integration system is known as a technique that can introduce multiple copies of a gene by homologous recombination with δ sequences present in large numbers on a yeast chromosome (for example, Appl. Microbiol. Biotechnol., 1997, Vol. 48, pp. 339-345; Biotechnology Letters, 2002, Vol. 24, pp. 1785-1790; Biotechnol. Prog., 1996, Vol. 12, pp. 16-21; Biotechnol. Prog., 1997, Vol. 13, pp. 368-373). These documents relate to that the introduction of multiple copies of a single expression gene to be expressed using integration with yeast δ sequences can result in increase in the level or improved efficiency of its expression, or can make the expression stable.